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    <meta charset="UTF-8">
    <title>Title</title>
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            border: 1px solid #000;
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<div style="height: 30px;text-align: center" >邮件系统</div>
<div style="height: 800px;display: flex">
    <div style="height: 795px;width: 100px;">邮件系统<br>
        <a href="/mails/write.html" target="contentFrame">写邮件</a><br>
        <a href="/mails/mail.html" target="contentFrame">发件箱</a><br>
        <a href="/mails/inbox.html" target="contentFrame">收件箱</a><br>
    </div>
    <iframe frameborder="0" src="/annex/first.png" name="contentFrame"
            width="1100" height="790"></iframe>
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